Conductive paste compound for via hole filling, printed circuit board which uses the conductive paste, and method of manufacturing the same

Abstract: A conductive paste compound for via hole filling includes a conductive filler at 80 to 92 weight percent with an average particle size of from 0.5 to 20µm and specific surface of from 0.1 to 1.5 m²/g, a liquid epoxy resin at 4.5 to 20 weight percent containing 2 or more epoxy groups with room temperature viscosity of 15 Pa·sec or less, and a hardener at 0.5 to 5 weight percent, wherein the viscosity is 2,000 Pa·sec or less and the volatile amount is 2.0 weight percent or less. A filling paste and a printed circuit board with use thereof are provided which can conduct an inner-via-hole connection between electrode layers without using a through-hole plating technique. The conductive paste comprises a metallic particle 103 such as copper, an epoxy resin, a hardener, and if necessary, a dispersant. The paste having low viscosity and low volatility under high shear is used to fill holes disposed in a laminated substrate 101. Then, this substrate is heated and pressurized together with copper foil 102 on both sides to attain a printed circuit board where both sides are electrically inner-via-hole connected.

Description: This invention relates to a conductive paste compound used for filling via holes and further relates to a double sided printed circuit board and a multilayer printed circuit board which uses the conductive paste. Recently, following the tendency of electronic devices towards high performance and compact size, a circuit board is required to have a high degree of lamination and high density. As a method of connecting layers within a substrate which can bond integrated circuits (IC) and components at the shortest distance, it is well-known that higher density can be attained through an inner-via-hole connection. In the case of a through-hole connection employed for a typical glass epoxy multilayer substrate, the connection is achieved by plating through-holes so that it was difficult to connect only the necessary layers. In addition, since the substrate comprises a top layer having an electrode land, this part can not be used to construct an electrode land of a surface mounting part. Therefore, due to these restrictions, the mounting density can not be easily enhanced. In order to solve these problems, methods other than using through-holes are employed, for example, a method of reducing through-holes by disposing holes halfway in a substrate, or a method of improving mounting density by filling conductive paste in through-holes and by further closing up holes disposed in the top layer of a substrate at a plating step. However, due to the complicated manufacturing processes, the above-mentioned methods have cost and quantity production problems. On the other hand, simply the necessary layers can be connected in the inner-via-hole connection, and since there is no through-holes in the top layer of the substrate, the mounting property is also excellent. When this connecting method is applied to a resin substrate, e.g. to a glass epoxy substrate, a double sided substrate establishes a connection by filling up through-holes with low-viscous solvent-type silver paste by means of a printing method and subsequent drying and hardening. However, the specific resistance value of this connection is as high as about 10 -3 Ω • cm, and the reliability in heat and shock resistance such as in the heat cycle is poor. In order to reduce the viscosity of the conductive paste, conventional methods employed were to use large particles in order to reduce the amount of conductive filler or the specific surface area, or to add a solvent with a low boiling point or a reactant type diluent. However, when an additive amount of conductive filler was reduced, or when large particles were used, the problem occurred that contact points between filler particles decreased and the resistance value of a via-hole-connection became so high that reliability could not be maintained in a test where thermal stress occurs, e.g., in the heat cycle test. With use of the method in which a solvent with a low boiling point or a reactant type diluent was added, these components volatilized and their content decreased enormously while being hardened through heat-pressing. Due to these volatile components, the base material either swelled or lost peel strength to the copper foils of the wiring. Furthermore, when a dispersant was not added, it was a problem that particle forms were limited to reduce viscosity, and that a printed filling of via holes was difficult even with use of pastes which are rather low in viscosity since the viscosity raised under high shear during the printing process. DE-A-41 25 879 discloses printed circuit boards and a process for their production. The boards comprise an isolating substrate coated on both sides with a copper layer having via holes, wherein an electrically conductive plating layer is provided on the inner walls of the via holes and on both surfaces of the substrate. The interior of the via holes is filled with the hardened product of an electrically non-conductive resin paste containing metal powder. The resin is preferably an epoxy resin of the novolac or bisphenol type and is not further specified. US-A-4,652,398 discloses an adhesive composition comprising as essential ingredients in its curable formulation, an epoxy resin, a soluble polyimide resin, a reactive solvent to dissolve the polyimide into the epoxy resin and, as a crosslinking agent, e.g., an aromatic compound containing polyhyroxy and polyalkenyl substitution. Optionally, the adhesive may also contain a conductive material. The epoxy resin which comprises from about 20% to about 35% by weight of the curable adhesive composition may be amongst many others a glycidyl ester of dimerized fatty acids. The metal content of the composition is at most 77,7%. The adhesive has utility in various bonding-applications where high temperature characteristics are important, for example, metal-to-metal bonding and plastic bonding in high temperature environments. An object of this invention is to solve the above-noted problems by providing a conductive paste having low viscosity and low volatility which can achieve a heat and shock resistant electrical connection between electrode layers through an inner-via-hole connection. Another object of this invention is to provide double sided printed circuit boards up to multilayer printed circuit boards including inner-via-hole connections using this paste. In order to accomplish these objects, a conductive paste for filling via holes as a first subject matter of this invention comprises (a) a conductive filler in an amount of 80 to 92 weight percent, having an average particle size of 0.5 to 20µm and a specific surface area of 0.1 to 1.5 m 2 /g, (b) a liquid epoxy resin in an amount of 4.5 to 19.5 weight percent, which contains two or more epoxy groups and comprises at least 10 weight percent of an epoxy resin which comprises glycidyl esterified dimer acids has a room temperature viscosity of 15 Pa•sec or less, and (c) a hardener in an amount of 0.5 to 5 weight percent, wherein the conductive paste has a viscosity of 2,000 Pa•sec or less and contains 2.0 weight percent or less of volatile components. It is preferable that the conductive paste further comprises a dispersant at 0.01 to 1.5 weight percent against 100 weight parts of the components (a) to (c). Furthermore, it is preferable that the conductive filler comprises at least one substance selected from gold, silver, palladium, copper, nickel, tin, and lead in the form of fine grains. It is preferable that the fine grains have a diameter of from 0.5 to 20 µm. It is also preferable that the conductive filler comprises copper with a surface oxygen concentration of 1.0 weight percent or less. In addition, it is preferable that the epoxy resin comprises at least one liquid epoxy resin selected from bisphenol A epoxy resins, bisphenol F epoxy resins, alicylic epoxy resins, amine epoxy resins and glycidyl esterificated epoxy resins. Furthermore, it is preferable that the liquid epoxy resin is a compound comprising 50 weight percent or less of one epoxy resin selected from bisphenol A and bisphenol F epoxy resins, and further comprising 50 weight percent or more of epoxy resin composed of glycidyl esterificated dimer acids with an epoxy equivalent of from 300 to 600 (g/eq). It is also preferable that the hardener is at least one hardener selected from amine hardeners such as dicyandiamide and carboxylic acid hydrazides, urea hardeners such as 3-(3,4-dichlorophenyl)-1,1-dimethyl urea, acid anhydride hardeners such as phthalic acid anhydride, methylnadic anhydride, pyromellitic acid anhydride, hexahydro phthalic acid anhydride, and aromatic amine (amine adduct) hardeners such as diamide diphenylmethane, diamide diphenylsulfonic acid. In addition, it is preferable that the hardener comprises a latent hardener. It is preferable that the dispersant comprises at least one dispersant selected from nonionic dispersants, e.g., ethylene oxides of higher fatty acids, propylene oxide addition ester compounds, sorbitan fatty acid esters, ethylene oxide of polyhydric alcohols such as sorbitan, propylene oxide addition ether compound, ethylene oxide of alkylbenzenes, propylene oxide additives, anionic dispersants, e.g., alkylbenzene sulfonic acid alkali salts, higher alcohol sulfate alkali salts, phosphate compounds, higher fatty acids, ethylene oxide of higher fatty acids, propylene oxide additives such as sulfate alkali salts, and quarternary ammonium salt type cationic dispersants. Next, a double sided printed circuit board of this invention is defined by the features of claim 11 and comprises an insulating base material disposed with inner via holes which are filled with the hot-pressed, hardened conductive paste as mentioned above, wherein the upper and lower electrode layers on the surface of the insulating base material are electrically connected. Next, a multilayer printed circuit board of this invention is defined by the features of claim 12 and comprises a plurality of insulating base material layers and two or more electrode layers, wherein each insulating base material is disposed with inner via holes filled with the hot-pressed, hardened conductive paste as mentioned above, wherein each electrode layer is electrically connected. It is preferable that the insulating base material comprises a composite material of an aramid fiber and a thermosetting resin. Furthermore, it is preferable that the insulating base material comprises a composite material of an aramid nonwoven fabric and an epoxy resin. A method of manufacturing a double sided printed circuit board of this invention is defined by the features of claim 15 and comprises the steps of: disposing a prepreg used for manufacturing a printed circuit board with via holes, filling the via holes with a conductive paste as mentioned above, placing the prepreg between copper foils on the upper and lower layers and providing heat and pressure, and forming circuits by etching the copper foils. A first method of manufacturing a multilayer printed circuit board of this invention is defined by the features of claim 16 and comprises the steps of: disposing a prepreg used for manufacturing a printed circuit board with via holes, filling the via holes with a conductive paste as mentioned above, positioning such a prepreg above and below a double sided printed circuit board, placing the prepregs between copper foils on the top layers, providing heat and pressure and etching the copper foils. A second method of manufacturing a multilayer printed circuit board of this invention as defined by the features of claim 16 comprises the steps of: disposing a prepreg used for manufacturing a printed circuit board with via holes, filling the via holes with a conductive paste as mentioned above, positioning such a prepreg above and below a double sided printed circuit board, providing heat and pressure, and forming circuits. It is preferable that the prepreg comprises a composite material of an aramid fiber and a thermosetting resin. Furthermore, it is preferable that the prepreg comprises a sheet composed of an aramid nonwoven fabric impregnated with a thermosetting epoxy resin. According to the embodiments of this invention, a non-solvent containing one-component conductive paste can be obtained by dispersing a specific conductive filler within an epoxy resin having specific characteristics. Furthermore, by filling via holes with this conductive paste, double sided printed circuit boards and multilayer printed circuit boards can be obtained which have a highly reliable via-hole-connection. First, the conductive filler will be explained. The conductive filler should preferably be contained in the conductive paste in high concentration. The reason for this is that, as mentioned above, conductive reliability must be maintained by raising the contact probability of the conductive filler particles to each other even if the substrates are distorted due to lower resistance of connected via holes or due to thermal and mechanical stress. In order to disperse the conductive filler with high concentration, the average particle size of the conductive filler should be preferably from 0.2 to 20 µm, and it should also have as small a specific surface area as possible. A preferred value for the specific surface is from 0.1 to 1.5 m 2 /g, and more preferably, from 0.1 to 1.0 m 2 /g. The average diameter can be measured by using a microscopic method or a light scattering method. Noble metals such as gold, silver, and palladium, or base metals such as copper, nickel, tin, and lead are illustrative examples of the conductive fillers, and it is also possible to use two different kinds or more of these metals simultaneously. As long as the conductive filler has the characteristics mentioned above, it can have a spherical shape or a flaked form etc. It is especially desirable to use copper powder as the conductive filler in view of migration control, economic supply, and price stability. But in general, copper powder is easily oxidized, and this oxidation of copper powder obstructs conductivity when it is used for filling via holes in this invention. Therefore, the oxygen concentration of the copper powder is preferably 1.0 weight % or less. Next, an epoxy resin of specific characteristics will be explained. In order to form a non-solvent containing one-component conductive compound, it is basically necessary to use a liquid resin as the epoxy resin. For the dispersion of the above-mentioned conductive filler in high concentration, viscosity of the epoxy resin must be 15 Pa•sec or less. If an epoxy resin with viscosity higher than this value is used, viscosity of the conductive paste formed becomes extremely high. This causes an inconvenience in that a via hole filling process can not take place when the viscosity of the conductive paste is 2,000 Pa•sec or higher. In addition, a preferred lower limit in viscosity of the epoxy resin is 0.2 Pa•sec. On the other hand, when this paste is heat-compressed after being filled into the via holes, the content of volatile constituents in the paste should preferably be controlled to prevent the volatile constituent from vaporizing and causing voids within the via hole filling structure or to prevent the prepreg from separating. The volatile amount is preferably as small as possible, and the above-noted problems can be avoided when the amount is 2.0 weight % or less. Examples of suitable epoxy resins are liquid epoxy resins including two or more epoxy radicals, for example, bisphenol A-type epoxy resin, bisphenol F-type epoxy resin, alicyclic epoxy resin, and amine epoxy resin. It is also possible to use liquid epoxy resins which are processed by molecular distillation to reduce the volatile amount. Above all, an epoxy resin composed of glycidyl esterificated dimer acids has low viscosity (e.g., 0.2 to 0.9 Pa• sec). Also, a hardened substance of this epoxy resin shows flexibility, and its relaxation effect against stress is large. Therefore, when this epoxy resin is blended within the epoxy resin at 10 weight part or more, reliability of the via hole structure becomes higher. Here, it is preferable that the liquid epoxy resin is a compound comprising 50 weight % or less of one epoxy resin selected from the group consisting of bisphenol A and bisphenol F, and further comprising 50 weight % or more of epoxy resin composed of glycidyl esterificated dimer acids with an epoxy equivalent of from 300 to 600 g/eq. As a result, reliability of the via hole structure becomes higher in which the epoxy resin has lower viscosity, the hardened substance shows flexibility, and the relaxation effect against stress is large. With regard to hardeners, any ordinary hardener can be used here. Generally used hardeners include amine-type hardeners such as dicyandiamide and carboxylic acid hydrazides, urea-type hardeners such as 3-(3,4-dichlorophenyl)-1,1-dimethyl urea, acid anhydride hardeners such as phthalic acid anhydride, methylnadic anhydride, pyromellitic acid anhydride, hexahydro phthalic acid anhydride, and aromatic amine-type (amine adduct) hardeners such as diamide diphenylmethane, diamide diphenylsulfonic acid. Above all, it is preferable to use a solid-type latent hardener, particularly in view of stability and workability of the compound. The solid-type latent hardener functions as follows. First, an amine component and an epoxy resin are reacted to some temperature (degree), grained as resin particles, and reactive radicals such as amine are shut up in a three-dimensional structure of polymer. When this is blended with an epoxy resin, the particles react partially on the surface. However, the reaction stops at this point, and they can be preserved at room temperature for a long time without changing the characteristics. Then, the particles start to melt or dissolve when heated at a temperature higher than predetermined, and the shut-up reactive radicals show up and start to react all at once, thereby hardening quickly. As for dispersants, any ordinary dispersant can be used. First, generally used dispersants include nonionic dispersants such as ethylene oxide of higher fatty acids, propylene oxide addition ester compounds, sorbitan fatty acid esters, ethylene oxide of polyhydric alcohols such as sorbitan, propylene oxide addition ether compounds, ethylene oxide of alkylbenzenes, propylene oxide additives etc. Secondly, anionic dispersants, e.g., alkylbenzene sulfonic acid alkali salts, higher alcohol sulfuric ester alkali salts, phosphate compounds, higher fatty acids, ethylene oxide of higher fatty acids, and sulfate alkali salts with propylene oxide additives can be used. Thirdly, quaternary ammonium salt type cationic dispersants can be used. In this instance, the dispersant is effective in reducing viscosity of the paste and adding fluidity to the metallic particles when shear is provided through increasing affinity between the metallic particle surface and an organic resin blended as binder in the paste. This function is seen in that the paste viscosity does not increase even under high shear in a screen printing process, and the paste can be filled easily into via holes disposed on a substrate. With regard to the prepreg, almost any substance can be used which is pressed to become thinner than the prepreg when hardened, e.g., glass epoxy, paper phenol, aramid epoxy etc. According to the above-mentioned methods of this invention, it is possible to obtain a conductive paste for via hole filling which has low viscosity and low volatility. Also, a double sided printed circuit board and a multilayer printed circuit board can be formed easily which have excellent reliability containing inner-via-hole connections. Here, the inner-via-hole connection is a method of obtaining connection between layers of the double sided and multilayer printed circuit board at any optional position. FIG. 1 is a cross-sectional view showing a structure of a double sided printed circuit board of an embodiment of this invention. FIG. 2 (a) - (d) are cross-sectional views showing a method of manufacturing a double sided printed circuit board of an embodiment of this invention. FIG. 3 (a) and (b) are cross-sectional views showing a method of manufacturing a multilayer printed circuit board of an embodiment of this invention. FIG. 4 (a) and (b) are cross-sectional views showing a method of manufacturing a multilayer printed circuit board of another embodiment of this invention. FIG. 5 is a schematic view of a conductive particle used for a printed circuit board of an embodiment of this invention. FIG. 6 is a graph showing the relationship between the change of via hole connection resistance value in a heat cycle test and the frequency of the test in an embodiment of this invention, in which the via hole connection is achieved with paste containing different epoxy resins in a printed circuit board. FIG. 7 is a graph showing the relationship between the change of via hole connection resistance value in a heat cycle test and the frequency of the test in an embodiment of this invention, in which the via hole connection is achieved with paste containing epoxy resin in different mixing ratios in a printed circuit board. FIG. 8 is a graph showing the relationship between the rotational frequency (shear) in an E-type viscometer and the viscosity of paste which includes a dispersant in different amounts. This invention will be described by referring to the following illustrative examples and attached figures. In particular, the following embodiments refer to a double sided printed circuit board using a conductive paste for via hole filling, a method of manufacturing the same, and a multilayer printed circuit board with use thereof. FIG. 1 is a cross-sectional view showing a structure of a double sided printed circuit board of an embodiment of this invention. A double sided printed circuit board 104 comprises a laminated base material 101, a copper foil 102 (In FIG. 1, a copper foil is formed into a circuit pattern), and a conductive via hole 103 in which a conductive paste has hardened. The major advantages of this invention are that a conductive paste can easily fill the holes due to its low viscosity and low volatility, and that the substrate has excellent reliability since the connection is achieved by a large amount of conductive filler. As laminated base material 101, it is possible to use laminated base materials which are widely known at present. Examples are a glass-epoxy base material, an aramid-epoxy base material, and a paper phenol base material. The base material is called "prepreg" before being heated and pressurized and comprises a glass cloth or a nonwoven fabric as a core material which is impregnated with an uncured resin. As for the conductive filler, almost any kind of noble metals such as gold and silver or base metals such as copper, tin, nickel or lead can be used. In addition, not only pure metals can be used, but also alloyed metals or those comprising a metal or an insulating core covered with a conductive material as shown in FIG. 5. In FIG. 5, 501 denotes a spherical core, and 502 denotes a conductive material which covers the core surface and serves as the conductive filler. FIG. 2 (a) to (d) are views showing a method of manufacturing a double sided printed circuit board of this invention. In FIG. 2, a laminated base material 201 is a prepreg. This prepreg is disposed with through-holes. Usually, a drill is used for this step, but a laser beam can be used for processing specific materials. FIG. 2 (b) shows a state in which FIG. 2 (a) is positioned between two copper foils 202. FIG. 2 (c) shows a state after FIG. 2 (b) is heated and pressurized. FIG. 2 (c) shows a state in which the through-holes disposed in the prepreg have an increased density of metal filling after being heated and pressurized. Due to compression, the prepreg has become thinner and the resin has hardened. The conductive paste 203 remains compressed. Conductor 103 of this state serves as an electrical connection between the upper and lower sides. FIG. 2 (d) shows a state after copper foil 202 on the surface is processed (e.g., etched) into circuit patterns. Copper foil 102 becomes a circuit conductor after being processed. After that, printed circuit boards used must be processed further prior to use, for example, applied with a solder resist, printed with letters and signs, or disposed with holes for insert components. FIGS. 3 (a) and (b) show a method of manufacturing a multilayer printed circuit board by repeatedly applying the above-mentioned method of manufacturing a double sided printed circuit board. FIG. 3 (a) shows a state in which a double sided circuit board 104 which serves as a core is positioned between substrates of FIG. 2 (a) having through-holes filled with conductive paste to which are further applied copper foils 202. When heated and pressurized from the upper and lower sides in this state, a multilayer printed circuit board of FIG. 3 (b) is obtained which has already established an inner-via-hole connection. When the copper foils on the upper and lower sides are patterned, a multilayer printed circuit board consisting of four layers is completed. After that, by repeating these steps, a multilayer printed circuit board with even more layers can be attained. In the method of manufacturing the multilayer printed circuit board of FIG. 3, the double sided printed circuit board used for the core was the double sided printed circuit board of this invention. However, this is not necessarily the case here. Instead, it is clear that a conventional through-hole double sided printed circuit board can be applied as well. In this case, it is better to fill the through-holes in advance. Here, the through-hole substrate indicates a resin substrate, and not only can the through-hole substrate be used but also a ceramic substrate etc. FIGS. 4 (a) and (b) show another method of manufacturing a multilayer printed circuit board. In FIG. 4 (a), a prepreg 201 which is filled with conductive paste 203 and is in the state before being heated and pressurized is positioned between two sheets of double sided printed circuit boards 104. When heated and pressurized in this state, a multilayer printed circuit board consisting of four layers of FIG. 4 (b) can be obtained. Not only a four-layer circuit board, but a multilayer printed circuit board consisting of more layers can be obtained by providing a plurality of double sided printed circuit boards, positioning the above-noted laminated substrates which are filled with a conductive filler and are in a state before being heated and pressurized between the double sided printed circuit boards, and heating and pressurizing. In the method of manufacturing the multilayer printed circuit board of FIG. 4, the double sided printed circuit board used was a double sided printed circuit board of this invention. However, this is not necessarily the case. Instead, a conventional through-hole double sided printed circuit board can be applied as well. In addition, not only can the through-hole substrate be used but also a ceramic substrate etc. Epoxy resins used in the following embodiments are indicated below. (1) bisphenol A-type epoxy resin (EPICOAT 828 manufactured by Yuka Shell Epoxy Co., Ltd., epoxy equivalent 184 ∼ 194 g/eq) (2) bisphenol F-type epoxy resin (EPICOAT 807 manufactured by Yuka Shell Epoxy Co., Ltd., epoxy equivalent 160 ∼ 175 g/eq) (3) alicylic epoxy resin (ST-1000 manufactured by Tohto Kasei Co., Ltd., epoxy equivalent 200 ∼ 220 g/eq) (4) amine-type epoxy resin (ELN-125 manufactured by Sumitomo Chemical Co., Ltd., epoxy equivalent 110 ∼ 130 g/eq) (5) epoxy resin composed of glycidyl esterificated dimer acids (epoxy equivalent 390 ∼ 470 g/eq). This resin is also called glycidyl ester type epoxy resin in these embodiments. Its formula is shown as Formula 1 below. Example 1 In a first embodiment of this invention shown in FIG. 1, a prepreg was an aramid epoxy sheet (TA-01 manufactured by Teijin Ltd.) of 200µm in thickness. This prepreg was applied on one side with a 20µm thick polyethylene terephthalate film by an adhesive and was disposed with through-holes of 0.2 mm in diameter by a drill. These through-holes were filled with via hole filling paste comprising metal particles of silver, gold, copper, and nickel in a spherical and flake form at 85 weight percent, a resin composition of bisphenol A-type epoxy resin (EPICOAT 828 manufactured by Yuka Shell Epoxy Co., Ltd.) at 3 weight %, an epoxy resin composed of glycidyl esterificated dimer acids (YD-171 manufactured by Tohto Kasei Co., Ltd.) at 9 weight %, and an amine adduct hardener (MY-24 manufactured by Ajinomoto Co., Ltd.) at 3 weight %. They were mixed and kneaded on three roles. Table 1 shows the form of the metal particle, the average size of the particle, the specific surface area, and the viscosity of the via hole filling paste of 0.5 rpm in an E-type viscometer at room temperature. Example Powder Form Average size of particle µm Specific surface area m 2 /g Paste viscosity Pa•sec Via resistance ×10 -5 Ω cm 1 silver spherical 0.2 3.14 2,300 6.3 2 0.5 1.35 1,050 1.2 3 1.0 0.62 620 0.3 4 2.0 0.30 480 0.3 5 5.0 0.16 320 0.4 6 10.0 0.10 100 0.6 7 flake 20.0 0.13 120 1.5 8 30.0 0.07 40 9.3 9 gold spherical 2.0 0.40 650 0.3 10 copper spherical 1.0 0.66 870 0.5 11 2.0 0.35 500 0.4 12 3.0 0.26 350 0.4 13 flake 10.0 0.20 400 1.5 14 nickel flake 5.0 0.32 680 2.7 The prepreg filled with the paste was applied with copper foil 102 of 30µm thickness on the upper and the lower sides. Then, this was formed into a double sided copper applied plate by heating and pressurizing 60 minutes at a temperature of 180°C and 50 Kg/cm 2 . When the paste viscosity was 2,000 Pa·sec or more, the filling in via holes was difficult to conduct due to high viscosity. On the double sided copper applied plate formed in this manner electrode patterns were formed by using a conventional etching technique. Table 1 shows the resistance value of the inner-via-hole-connection conducted by using the respective paste. In all cases, a low resistance connection was obtained in which the resistivity value was ten times or less of the specific resistance value of the conductor although the paste included an insulating resin. The specific resistance value of each metal is (silver) 1.6 × 10 -6 Ω cm, (gold) 2.3 × 10 -6 Ω cm, and (nickel) 6.8 × 10 -6 Ωcm. Example 2 As in the first embodiment, the prepreg of a second embodiment had through-holes formed with 0.2 mm in diameter, and these through-holes were filled with conductive paste comprising metal particles of copper powder with an average particle size of 2µm at 85 weight %, a resin selected from bisphenol A-type epoxy resin (EPICOAT 828 manufactured by Yuka Shell Epoxy Co., Ltd.), bisphenol F-type epoxy resin (EPICOAT 807 manufactured by Yuka Shell Epoxy Co., Ltd.), alicylic epoxy resin (ST-1000 manufactured by Tohto Kasei Co., Ltd.), amine-type epoxy resin (ELN-125 manufactured by Sumitomo Chemical Co., Ltd.), and epoxy resin composed of glycidyl esterificated dimer acids at 12 weight %, and an amine adduct hardener (PN-23 manufactured by Ajinomoto Co., Ltd.) at 3 weight %. They were mixed and kneaded on three roles. Table 2 shows the viscosity of the via hole filling paste at room temperature, the decrease of the paste weight when heated to 300°C at a rate of 10°C/min., and the volatile amount. In all cases, the volatile amount was 2 weight % or less, and there was no swelling found during the substrate formation. Example Resin Paste viscosity Pa•sec Volatile amount WT% Via resistance ×10 -5 Ω cm 1 bisphenol A-type 1,500 0.16 0.4 2 bisphenol F-type 800 0.15 0.4 3 alicyclic epoxy 200 0.30 0.7 4 amine-type epoxy 60 0.30 0.6 5 glycidyl ester type 80 0.40 0.4 The prepreg having holes was filled with the paste and copper foil 102 was applied on the upper and the lower sides. Then, this was formed into a double sided copper applied plate by heating and pressurizing 60 minutes at 180°C and 50 Kg/cm 2 . Electrode patterns were formed by using a conventional etching technique. Table 2 shows the resistance value of the inner-via-hole-connection conducted by using the respective paste. In all cases, a low resistance connection was obtained in which the resistivity value was ten times or less of the specific resistance value of the conductor although the paste included an insulating resin. Furthermore, FIG. 6 shows the change of the via hole connection resistance value in a heat cycle test (-55°C ∼ 125°C, 30 minutes each) up to 1000 cycles. The change of the via hole resistance value in each paste sample was 10 percent or less against the initial resistance value, so the reliability of the via hole was not ruined. In particular, the glycidyl ester type showed a rate of change at 1 % or less. Due to the flexibility of the resin, only a slight change occurred against a thermal shock, and the reliability of the via-hole-connection was extremely good. Example 3 As in the first embodiment, the prepreg of a third embodiment had through-holes formed with 0.2 mm in diameter, and these through-holes were filled with conductive paste comprising metal particles of copper powder with an average particle size of 2µm at 85 weight %, a blended resin consisting of one resin selected from Group A (bisphenol A-type epoxy resin, bisphenol F-type epoxy resin) and one resin selected from Group B (alicylic epoxy resin, amine-type epoxy resin, or epoxy resin composed of glycidyl esterificated dimer acids) at 12 gross weight %, and an amine adduct hardener (MY-24) at 3 weight %. They were mixed and kneaded on three roles. Table 3 shows the viscosity of the via hole filling paste of 0.5 rpm in an E-type viscometer at room temperature. Example Resin A WT% Resin B WT% Paste viscosity Pa•sec Via resistance ×10 -5 Ω cm 1 bisphenol A-type 50 alicyclic 50 720 0.8 2 50 amine-type 50 540 0.6 3 75 glycidyl ester 25 1,060 0.5 5 50 50 660 0.5 6 25 75 380 0.4 7 bisphenol F-type 75 glycidyl ester 25 620 0.5 9 50 50 400 0.4 10 25 75 220 0.4 The prepreg filled with this paste was applied with copper foil 102 on the upper and the lower sides. Then, this was formed into a double sided copper applied plate by heating and pressurizing 60 minutes at 180°C and 50 Kg/cm 2 . Electrode patterns were formed by using a conventional etching technique. Table 3 shows the resistance value of the inner-via-hole-connection conducted by using the respective paste. In all cases, a low resistance connection was obtained in which the resistivity value was ten times or less of the specific resistance value of the conductor although the paste included an insulating resin. Furthermore, FIG. 7 shows the change of the via hole resistance value in a heat cycle test (-55°C ∼ 125°C, 30 minutes each) up to 1000 cycles. The change of the via hole resistance value in each paste sample was 10 percent or less against the initial resistance value, so the reliability of the via hole was not ruined. In particular, the glycidyl ester mixed type showed a rate of change at 1 % or less at the amount content of 50 weight % or more, thereby maintaining the reliability of the via-hole-connection. Example 4 As in the first embodiment, the prepreg of a fourth embodiment had through-holes formed with 0.2 mm in diameter, and these through-holes were filled with conductive paste comprising metal particles of silver powder or copper powder with an average particle size of 2µm at 75 to 92.5 weight %, a blended resin consisting of bisphenol A-type epoxy resin and epoxy resin composed of glycidyl esterificated dimer acids in 25 to 75 weight ratio at 4.5 to 22 gross weight %, and an amine adduct hardener (MY-24) at 3 weight %. They were mixed and kneaded on three roles. Table 4 shows the viscosity of the via hole filling paste at room temperature. Example Metal weight WT% Resin WT% Paste viscosity Pa•sec Via resistance ×10 -5 Ω cm 1 silver 80.0 bisphenol A (25WT%) + glycidyl ester (75WT%) 17 250 0.7 2 85.0 12 480 0.4 3 copper 75.0 bisphenol A (25WT%) + glycidyl ester (75WT%) 22 40 4.8 4 80.0 17 180 0.8 5 85.0 12 380 0.4 6 90.0 7 890 0.4 7 92.0 5 1,450 0.3 8 92.5 4.5 2,310 0.3 The prepreg filled with this paste was applied with copper foil 102 on the upper and the lower sides. Then, this was formed into a double sided copper applied plate by heating and pressurizing 60 minutes at 180°C and 50 Kg/cm 2 . Electrode patterns were formed by using a conventional etching technique. Table 4 shows the resistance value of the inner-via-hole-connection conducted by using the respective paste. In all cases, a low resistance connection was obtained in which the resistivity value was ten times or less of the specific resistance value of the conductor although the paste included an insulating resin. On the other hand, when the metal content was 80 weight % or less, the via hole resistance value was large due to the lack of the conductive metal. Furthermore, when the metal content was 92 weight % or more, the via hole filling was difficult to conduct due to high paste viscosity. Example 5 As in the first embodiment, the prepreg of a fifth embodiment had through-holes formed with 0.2 mm in diameter, and these through-holes were filled with conductive paste comprising metal particles of silver powder or copper powder with an average particle size of 2µm at 85 weight %, a blended resin consisting of bisphenol A-type epoxy resin and epoxy resin composed of glycidyl esterificated dimer acids in 25 to 75 weight ratio at 12 gross weight %, and dicyandiamide (DICY7 manufactured by Yuka Shell Epoxy Co., Ltd.) or acid anhydride (RIKACID MH manufactured by New Japan Chemical Co., Ltd.) or amine adduct hardener (PN-23, MY-24 manufactured by Ajinomoto Co., Ltd.) at 3 weight %. They were mixed and kneaded on three roles. Table 5 shows the viscosity of the via hole filling paste at room temperature and the volatile amount of the paste when heated to 300°C at a rate of 10°C/min. Example Metal Hardener Paste viscosity Pa·sec Volatile amount WT% Via resistance × 10 -5 Ω cm 1 silver dicyandiamide 520 0.32 0.4 2 acid anhydride 200 1.50 0.5 3 amine adduct(MY-24) 620 0.50 0.4 4 amine adduct(PN-23) 600 0.42 0.4 5 copper dicyandiamide 380 0.35 0.4 6 acid anhydride 120 1.56 0.5 7 amine adduct(MY-24) 430 0.50 0.4 8 amine adduct(PN-23) 450 0.40 0.4 The prepreg filled with this paste was applied with copper foil 102 on the upper and the lower sides. Then, this was formed into a double sided copper applied plate by heating and pressurizing 60 minutes at 180°C and 50 Kg/cm 2 . Electrode patterns were formed by using a conventional etching technique. Table 5 shows the resistance value of the inner-via-hole-connection conducted by using the respective paste. Regardless of the hardener type, a low resistance connection was obtained in which the resistivity value was ten times or less of the specific resistance value of the conductor although the paste included an insulating resin. Example 6 As in the first embodiment, the prepreg of a sixth embodiment had through-holes formed with 0.2 mm in diameter, and these through-holes were filled with conductive paste comprising metal particles of copper powder with an average particle size of 2µm at 85 weight %, a blended resin consisting of bisphenol A-type epoxy resin and epoxy resin composed of glycidyl esterificated dimer acids in 25 to 75 weight ratio at 8 to 14.5 gross weight %, and phthalic anhydride or amine adduct type hardener (MY-24) at 0.5 to 7 weight %. They were mixed and kneaded and mixed on three roles. Table 6 shows the viscosity of the via hole filling paste at room temperature and the volatile amount of the paste when heated to 300°C at a rate of 10°C/min. Example Hardener Amount of hardener WT% Paste viscosity Pa·sec Volatile amount WT% Via resistance ×10 -5 Ω cm 1 acid anhydride 0.5 160 0.60 0.8 2 3.0 120 1.56 0.5 3 5.0 120 1.95 0.4 4 7.0 110 2.34 - 5 amine adduct 0.5 180 0.40 0.4 6 3.0 450 0.40 0.4 7 5.0 850 0.38 0.5 8 7.0 2,400 0.35 20.0 The prepreg filled with this paste was applied with copper foil 102 on the upper and the lower sides. Then, this was formed into a double sided copper applied plate by heating and pressurizing 60 minutes at 180°C and 50 Kg/cm 2 . Electrode patterns were formed by using a conventional etching technique. Table 6 shows the resistance value of the inner-via-hole-connection conducted by using the respective paste. Regardless of the hardener type, a low resistance connection was obtained in which the resistivity value was ten times or less of the specific resistance value of the conductor although the paste included an insulating resin. However, when too much solid powder such as an amine adduct hardener was added, the viscosity increased, so the via hole filling was difficult to conduct by using the paste added with a hardener of 5 weight % or more. Furthermore, when the paste using a hardener with high volatility such as acid anhydride had a volatile amount of more than 2 weight %, a double sided copper applied plate of satisfactory quality was not obtained due to swelling of the substrate and the electrode. Example 7 As in the first embodiment, the prepreg of a seventh embodiment had through-holes formed with 0.2 mm in diameter, and these through-holes were filled with conductive paste comprising metal particles of copper powder with an average particle size of 2µm having different surface oxygen amounts at 85 weight %, a blended resin consisting of bisphenol A-type epoxy resin and epoxy resin composed of glycidyl esterificated dimer acids in 25 to 75 weight ratio at 12 gross weight %, and an amine adduct hardener (MY-24) at 3 weight %. They were mixed and kneaded on three roles. Table 7 shows the viscosity of the via hole filling paste at room temperature. Example Surface oxygen amount WT% Paste viscosity Pa·sec Via resistance ×10 -5 Ω cm 1 0.2 430 0.3 2 0.5 450 0.4 3 1.0 500 0.8 4 1.2 680 2.5 The prepreg filled with this paste was applied with copper foil 102 on the upper and the lower sides. Then, this was formed into a double sided copper applied plate by heating and pressurizing 60 minutes at 180°C and 50 Kg/cm 2 . Electrode patterns were formed by using a conventional etching technique. Table 7 shows the resistance value of the inner-via-hole-connection conducted by using the respective paste. When using a paste with the surface oxygen concentration of copper at 1.0 weight % or less, a low resistance connection was obtained in which the resistivity value was ten times or less of the specific resistance value of the conductor although the paste included an insulating resin. However, along with the increase in the surface oxygen concentration, the resistance of the inner-via-hole connection became higher. As soon as this amount exceeded 1.0 weight %, the resistance value increased radically. As a result, a low resistance connection was not obtained in which the resistivity value was ten times or less of the specific resistance value of the conductor. Example 8 The conductive paste used here comprised metal particles of copper powder with an average particle size of 2µm at 85 weight %, a blended resin consisting of bisphenol A-type epoxy resin and epoxy resin composed of glycidyl esterificated dimer acids in 25 to 75 weight ratio at 12 gross weight %, and an amine adduct hardener (MY-24) at 3 weight %. The components had been mixed and kneaded on three roles. An aramid-epoxy prepreg was disposed with through-holes of 0.2 mm in diameter which were filled with the above-mentioned paste. This aramid-epoxy prepreg was positioned between two sheets of aramid-epoxy double sided substrates having patterns formed for the connection resistance measurement. Then, this was formed into a four-layer printed circuit board by heating and pressurizing 60 minutes at 180°C and 50 Kg/cm 2 . The inner-via-hole connection resistance formed between the second and the third layer of the four-layer substrate showed the same resistance value as in the first embodiment. Also, a six-layer printed circuit board attained the same reliability with the same connection resistance value as in the four-layer printed circuit board. Furthermore, the above-mentioned paste was used for manufacturing an aramid-epoxy double sided substrate with the same method as in Example 1. One sheet of this substrate was positioned between two sheets of aramid-epoxy prepregs having through-holes of 0.2 mm in diameter filled with the above-noted paste. In the method of forming a four-layer printed circuit board by heating and pressurizing 60 minutes at 180°C and 50 Kg/cm 2 , equivalent resistance was obtained for the via-hole-connection. In addition, the same reliability was obtained in a six-layer printed circuit board manufactured by using this method. A ceramic substrate with circuits was used instead of the aramid-epoxy double sided substrate manufactured by the same method as in Example 1, and the same reliability was obtained for the via-hole-connection resistance. Example 9 In a ninth embodiment of this invention shown in FIG. 1, the prepreg was an aramid-epoxy sheet (TA-01 manufactured by Teijin Ltd.) of 200µm in thickness. This prepreg was applied on one side with a 20µm thick polyethylene terephthalate film by an adhesive and was disposed with through-holes of 0.2 mm in diameter by a drill. These through-holes were filled with via hole filling paste comprising metal particles of silver, gold, copper and nickel in a spherical and flake form at 85 weight %, a resin composition of bisphenol A-type epoxy resin (EPICOAT 828 manufactured by Yuka Shell Epoxy Co., Ltd.) at 3 weight %, an epoxy resin composed of glycidyl esterificated dimer acids (YD-171 manufactured by Tohto Kasei Co., Ltd.) at 9 weight %, an amine adduct hardener (MY-24 manufactured by Ajinomoto Co., Ltd.) at 3 weight %, and a dispersant at 0.01 to 2.0 weight % against the gross amount of the metal, the resin, and the hardener comprising one dispersant selected from polyoxyethylene sorbitan fatty acid ester (nonionic surface active agent "SOLGEN" TW manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.), phosphate (anion surface active agent "PLYSURF" manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.), and cation surface active agent "CATIOGEN" (manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.). They were mixed and kneaded on three roles. Table 8 shows the form of the metal particles, the average size of the particles, the specific surface area, the type of the dispersant and its compounded amount (weight %), and the viscosity of the via hole filling paste of 0.5 rpm in an E-type viscometer at room temperature. Example Powder Form Average size of particle µm Specific surface area m 2 /g Dispersant Compounded amount at weight % Paste viscosity Pa•sec Via resistance ×10 -5 Ω cm 1 silver spherical 0.1 5.83 PLYSURF 0.20 2,350 4.7 2 0.2 3.14 1,620 1.2 3 0.5 1.35 850 0.4 4 1.0 0.62 480 0.3 5 2.0 0.30 390 0.3 6 5.0 0.16 270 0.4 7 10.0 0.10 80 0.6 8 flake 20.0 0.13 90 1.3 9 30.0 0.07 30 10.3 10 gold spherical 2.0 0.40 470 0.3 11 copper spherical 1.0 0.66 820 0.5 12 2.0 0.35 0 500 0.4 13 0.01 470 0.4 14 0.20 420 0.4 15 0.50 410 0.5 16 1.50 380 0.8 17 2.00 350 2.3 18 SOLGEN TW 0.20 450 0.4 19 CATIOGEN 0.20 460 0.4 20 3.0 0.26 PLYSURF 0.20 300 0.4 21 flake 10.0 0.20 370 1.5 22 nickel flake 5.0 0.32 610 2.0 The prepreg filled with the paste was applied with copper foil 102 of 35µm thick on the upper and the lower sides. Then, this was formed into a double sided copper applied plate by heating and pressurizing 60 minutes at 180°C and 50 Kg/cm 2 . When the paste viscosity was 2,000 Pa•sec or more, which is beyond the limits of claims of this invention, the filling into via holes was difficult to conduct due to high viscosity. On the double sided copper applied plate formed in this manner electrode patterns were formed by using a conventional etching technique. Table 8 shows the resistance value of the inner-via-hole-connection conducted by using the respective paste. In all cases, a low resistance connection was obtained in which the resistivity value was ten times or less of the specific resistance value of the conductor although the paste included an insulating resin. The specific resistance value of each metal used in this embodiment is: silver (1.6 × 10 -6 Ωcm), gold (2.3 × 10 -6 Ωcm), copper (1.7 × 10 -6 Ωcm), and nickel (6.8 × 10 -6 Ωcm). Furthermore, the change of the paste viscosity is shown in FIG. 8 in which shear is provided to the paste by changing the frequency with an E-type viscometer. It is desirable for a filling paste that the viscosity decreases under high shear in a printing process compared to under the condition of no shear. In the examples without dispersant added the viscosity increased when provided with shear, so it was difficult to fill the paste. On the other hand, the examples with dispersant added made a small increase in the viscosity under high shear, and for those added more than 0.5 weight part the viscosity decreased under high shear. Accordingly, the filling became easier to conduct. However, the examples with dispersant added 2.0 weight % or more had the connection of the metal particles disturbed, so the via connection resistance increased. Example 10 As in the ninth embodiment, the prepreg of a tenth embodiment had through-holes formed with 0.2 mm in diameter, and these through-holes were filled with conductive paste comprising metal particles of copper powder with an average size of 2µm at 85 weight %, a resin selected from bisphenol A-type epoxy resin, bisphenol F-type epoxy resin (EPICOAT 807 manufactured by Yuka Shell Epoxy Co., Ltd.), alicylic epoxy resin (ST-1000 manufactured by Tohto Kasei Co., Ltd.), amine type epoxy resin (ELN-125 manufactured by Sumitomo Chemical Co., Ltd.), and an epoxy resin composed of glycidyl esterificated dimer acids at 12 weight %, an amine adduct hardener (PN-23 manufactured by Ajinomoto Co., Ltd.) at 3 weight %, and a dispersant containing phosphate ("PLYSURF" manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) contained at 0.2 weight % against the gross amount of the metal, the resin, and the hardener. They were mixed and kneaded on three roles. Table 9 shows the viscosity of the via hole filling paste at room temperature and the decrease of the volatile amount of the paste when heated to 300°C at a rate of 10°C/min. In all cases, the volatile amount of the paste was 2 weight % or less, and there was no swelling found during the substrate manufacturing process. Example Resin Paste viscosity Pa•sec Volatile amount WT% Via resistance ×10 -5 Ω cm 1 bisphenol A-type 1,350 0.16 0.4 2 bisphenol F-type 720 0.15 0.4 3 alicyclic epoxy 180 0.30 0.8 4 amine-type epoxy 50 0.30 0.6 5 glycidyl ester type 60 0.40 0.6 The prepreg having holes was filled with the paste and applied with copper foil 102 on the upper and the lower sides. Then, this was formed into a double sided copper applied plate by heating and pressurizing 60 minutes at 180°C and 50 Kg/cm 2 . Electrode patterns were formed by using a conventional etching technique. Table 9 shows the resistance value of the inner-via-hole-connection conducted by using the respective paste. In all cases, a low resistance connection was obtained in which the resistivity value was ten times or less of the specific resistance value of the conductor although the paste included an insulating resin. Example 11 As in the eighth embodiment, the prepreg of an eleventh embodiment had through-holes formed with 0.2 mm in diameter, and these through-holes were filled with conductive paste comprising metal particles of copper powder with an average particle size of 2µm at 85 weight %, a blended resin consisting of one epoxy resin selected from Group A (bisphenol A-type epoxy resin, bisphenol F-type epoxy resin) and one selected from Group B (alicylic epoxy resin, amine-type epoxy resin, epoxy resin composed of glycidyl esterificated dimer acids) at 12 gross weight %, an amine adduct hardener (MY-24) at 3 weight %, and a dispersant containing phosphate ("PLYSURF" manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) at 0.2 weight % against the gross amount of the metal, the resin, and the hardener. They were mixed and kneaded on three roles. An aramid-epoxy prepreg comprised a prepreg disposed with through-holes of 0.2 mm in diameter which were filled with the above-mentioned paste. Table 10 shows the viscosity of the via hole filling paste of 0.5 rpm in an E-type viscometer at room temperature. Example Resin A WT% Resin B WT% Paste viscosity Pa•sec Via resistance ×10 -5 Ω cm 1 bisphenol A-type 50 alicyclic 50 650 0.8 2 50 amine-type 50 460 0.5 3 75 glycidyl ester 25 870 0.5 5 50 50 600 0.6 6 25 75 340 0.5 7 bisphenol F-type 75 glycidyl ester 25 570 0.5 9 50 50 380 0.4 10 25 75 200 0.5 The prepreg filled with this paste was applied with copper foil 102 on the upper and the lower sides. Then, this was formed into a double sided copper applied plate by heating and pressurizing 60 minutes at 180°C and 50 Kg/cm 2 . Electrode patterns were formed by using a conventional etching technique. Table 10 shows the resistance value of the inner-via-hole-connection conducted by using the respective paste. A low resistance connection was obtained in which the resistivity value was ten times or less of the specific resistance value of the conductor although the paste included an insulating resin. Example 12 As in the eighth embodiment, the prepreg of a twelfth embodiment had through-holes formed with 0.2 mm in diameter, and these through-holes were filled with conductive paste comprising metal particles of silver powder or copper powder with an average particle size of 2µm at 75 to 93.0 weight %, a blended resin consisting of bisphenol A-type epoxy resin and epoxy resin composed of glycidyl esterificated dimer acids in 25 to 75 weight ratio at 4.0 to 22 gross weight %, an amine adduct hardener (MY-24) at 3 weight %, and a dispersant containing phosphate ("PLYSURF" manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) at 0.2 weight % against the gross amount of the metal, the resin, and the hardener. They were mixed and kneaded on three roles. Table 11 shows the viscosity of the via hole filling paste at room temperature. Example Metal weight WT% Resin WT% Paste viscosity Pa•sec Via resistance ×10 -5 Ω cm 1 silver 80.0 bisphenol A (25WT%) + glycidyl ester (75WT%) 17 250 0.7 2 85.0 12 480 0.4 3 copper 75.0 bisphenol A (25WT%) + glycidyl ester (75WT%) 22 40 5.0 4 80.0 17 180 0.7 5 85.0 12 380 0.4 6 90.0 7 890 0.4 7 92.0 5 1,450 0.3 8 92.5 4.5 1,880 0.4 9 93.0 4 2,680 - The prepreg filled with this paste was applied with copper foil 102 on the upper and the lower sides. Then, this was formed into a double sided copper applied plate by heating and pressurizing 60 minutes at 180°C and 50 Kg/cm 2 . Electrode patterns were formed by using a conventional etching technique. Table 11 shows the resistance value of the inner-via-hole-connection conducted by using the respective paste. A low resistance connection was obtained in which the resistivity value was ten times or less of the specific resistance value of the conductor although the paste included an insulating resin. However, in the compositions beyond the limits of claims of this invention, when the metal content was 80 weight % or less, the via hole resistance value was large due to the lack of the conductive metal. Furthermore, when the metal content was more than 92 weight %, the via hole filling was difficult to conduct due to high paste viscosity. Example 13 As in the eighth embodiment, the prepreg of a thirteenth embodiment had through-holes formed with 0.2 mm in diameter, and these through-holes were filled with conductive paste comprising metal particles of silver powder or copper powder with an average particle size of 2µm at 85 weight %, a blended resin consisting of bisphenol A-type epoxy resin and epoxy resin composed of glycidyl esterificated dimer acids in 25 to 75 weight ratio at 12 gross weight %, a hardener selected from dicyandiamide (DICY7 manufactured by Yuka Shell Epoxy Co., Ltd.) or acid anhydride (RIKACID MH manufactured by New Japan Chemical Co., Ltd.) or amine adduct hardener (PN-23, MY-24 manufactured by Ajinomoto Co., Ltd.) at 3 weight %, and a dispersant containing phosphate ("PLYSURF" manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) at 0.2 weight % against the gross amount of the metal, the resin, and the hardener. They were mixed and kneaded on three roles. Table 12 shows the viscosity of the via hole filling paste at room temperature (25°C) and the volatile amount of the paste when heated to 300°C at a rate of 10°C/min. Example Metal Hardener Paste viscosity Pa•sec Volatile amount WT% Via resistance × 10 -5 Ω cm 1 silver dicyandiamide 470 0.32 0.4 2 acid anhydride 180 1.50 0.4 3 amine adduct(MY-24) 540 0.50 0.4 4 amine adduct(PN-23) 520 0.42 0.4 5 copper dicyandiamide 320 0.35 0.4 6 acid anhydride 100 1.56 0.5 7 amine adduct(MY-24) 380 0.50 0.4 8 amine adduct(PN-23) 380 0.40 0.4 The prepreg filled with this paste was applied with copper foil 102 on the upper and the lower sides. Then, this was formed into a double sided copper applied plate by heating and pressurizing 60 minutes at 180°C and 50 Kg/cm 2 . Electrode patterns were formed by using a conventional etching technique. Table 12 shows the resistance value of the inner-via-hole-connection conducted by using the respective paste. Regardless of the hardener type, a low resistance connection was obtained in which the resistivity value was ten times or less of the specific resistance value of the conductor although the paste included an insulating resin. Example 14 As in the eighth embodiment, the prepreg of a fourteenth embodiment had through-holes formed with 0.2 mm in diameter, and these through-holes were filled with conductive paste comprising metal particles of silver powder or copper powder with an average particle size of 2µm at 85 weight %, a blended resin consisting of bisphenol A-type epoxy resin and epoxy resin composed of glycidyl esterificated dimer acids in 25 to 75 weight ratio at 8 to 14.5 gross weight %, a phthalic anhydride or amine adduct hardener (MY-24) at 0.5 to 7 weight %, and a dispersant containing phosphate ("PLYSURF" manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) at 0.2 weight % against the gross amount of the metal, the resin, and the hardener. They were mixed and kneaded on three roles. Table 13 shows the viscosity of the via hole filling paste at room temperature and the volatile amount of the paste when heated to 300°C at a rate of 10°C/min. Example Hardener Amount of hardener WT% Paste viscosity Pa•sec Volatile amount WT% Via resistance ×10 -5 Ω cm 1 acid anhydride 0.5 140 0.60 0.9 2 3.0 100 1.58 0.5 3 5.0 110 1.95 0.4 4 7.0 110 2.34 - 5 amine adduct 0.5 170 0.40 0.5 6 3.0 410 0.40 0.5 7 5.0 720 0.38 0.5 8 7.0 2,100 0.35 15.0 The prepreg filled with this paste was applied with copper foil 102 on the upper and the lower sides. Then, this was formed into a double sided copper applied plate by heating and pressurizing 60 minutes at 180°C and 50 Kg/cm 2 . Electrode patterns were formed by using a conventional etching technique. Table 13 shows the resistance value of the inner-via-hole-connection conducted by using the respective paste. Regardless of the weight ratio of the hardener, a low resistance connection was obtained in which the resistivity value was ten times or less of the specific resistance value of the conductor although the paste included an insulating resin. However, when too much solid powder such as an amine adduct hardener was added, the viscosity increased, so the via hole filling was difficult to conduct by using the paste added with a hardener of 5 weight % or more. Furthermore, when the paste using a hardener with high volatility such as acid anhydride had a volatile amount of more than 2 weight %, a double sided copper applied plate of satisfactory quality was not obtained due to swelling of the substrate and the electrode. Example 15 As in the eighth embodiment, the prepreg of a fifteenth embodiment had through-holes formed with 0.2 mm in diameter, and these through-holes were filled with conductive paste comprising metal particles of copper powder with an average particle size of 2µm having different surface oxygen amounts at 85 weight %, a blended resin consisting of bisphenol A-type epoxy resin and epoxy resin composed of glycidyl esterificated dimer acids in 25 to 75 weight ratio at 12 gross weight %, an amine adduct hardener (MY-24) at 3 weight %, and a dispersant containing phosphate ("PLYSURF" manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) at 0.2 weight % against the gross amount of the metal, the resin, and the hardener. They were mixed and kneaded on three roles. Table 14 shows the viscosity of the via hole filling paste at room temperature. Example Surface oxygen amount WT% Paste viscosity Pa•sec Via resistance ×10 -5 Ω cm 1 0.2 400 0.3 2 0.5 410 0.3 3 1.0 430 0.5 4 1.2 580 2.5 The prepreg filled with this paste was applied with copper foil 102 on the upper and the lower sides. Then, this was formed into a double sided copper applied plate by heating and pressurizing 60 minutes at 180°C and 50 Kg/cm 2 . Electrode patterns were formed by using a conventional etching technique. Table 14 shows the resistance value of the inner-via-hole-connection conducted by using the respective paste. When using paste with the surface oxygen concentration of copper at 1.0 weight % or less, a low resistance connection was obtained in which the resistivity value was ten times or less of the specific resistance value of the conductor although the paste included an insulating resin. However, along with the increase in the surface oxygen concentration, the resistance of the inner-via-hole connection became higher. As soon as this amount exceeded 1.0 weight %, the resistance value increased radically. As a result, a low resistance connection was not obtained in which the resistivity value was ten times or less of the specific resistance value of the conductor. Example 16 Conductive paste used here comprised metal particles of copper powder with an average particle size of 2µm at 85 weight %, a blended resin consisting of bisphenol A-type epoxy resin and epoxy resin composed of glycidyl esterificated dimer acids in 25 to 75 weight ratio at 12 gross weight %, an amine adduct hardener (MY-24) at 3 weight %, and a dispersant containing phosphate ("PLYSURF" manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) at 0.2 weight % against the gross amount of the metal, the resin, and the hardener. They were mixed and kneaded on three roles. An aramid-epoxy prepreg was disposed with through-holes of 0.2 mm in diameter which were filled with the above-mentioned paste. This aramid-epoxy prepreg was positioned between two sheets of aramid-epoxy double sided substrates having patterns formed for the connection resistance measurement. Then, this was formed into a four-layer printed circuit board by heating and pressurizing 60 minutes at 180°C and 50 Kg/cm 2 . The inner-via-hole connection resistance formed between the second and the third layer of the four-layer substrate showed the same resistance value as in the first embodiment. Also, a six-layer printed circuit board attained the same reliability with the same connection resistance value as in the four-layer printed circuit board. Furthermore, the above-mentioned paste was used for manufacturing an aramid-epoxy double sided substrate with the same method as in Example 8. One sheet of this substrate was positioned between two sheets of aramid-epoxy prepregs having through-holes of 0.2 mm in diameter filled with the above-noted paste. In the method of forming a four-layer printed circuit board by heating and pressurizing 60 minutes at 180°C and 50 Kg/cm 2 , equivalent resistance was obtained for the via-hole-connection. In addition, the same reliability was obtained in a six-layer printed circuit board manufactured by using this method. A ceramic substrate with circuits was used instead of the aramid-epoxy double sided substrate manufactured by the same method as in Example 8, and the same reliability was obtained for the via-hole-connection resistance. As described above, similar to a double sided printed circuit board, a multilayer printed circuit board, and methods of manufacturing the same with use of the via hole filling paste of this invention, and a double sided printed circuit board with inner via holes can be attained without using a through-hole plating technique. Furthermore, this circuit board can be formed into a multilayer printed circuit board easily.

Claims: A conductive paste for filling via holes comprising (a) a conductive filler in an amount of 80 to 92 weight percent having an average particle size of 0.5 to 20µm and a specific surface area of 0.1 to 1.5 m 2 /g, (b) a liquid epoxy resin in an amount of 4.5 to 19.5 weight percent which contains two or more epoxy groups and comprises at least 10 weight percent of an epoxy resin which comprises glycidyl esterified dimer acids and has a room temperature viscosity of 15 Pa•sec or less, and (c) a hardener in an amount of 0.5 to 5 weight percent, wherein the conductive paste has a viscosity of 2,000 Pa•sec or less, and contains 2.0 weight percent or less of volatile components. The conductive paste of claim 1, further comprising a dispersant in an amount of 0.01 to 1.5 weight percent against 100 weight parts of said components (a) to (c). The conductive paste of claim 1 or 2, wherein said conductive filler comprises at least one substance selected from gold, silver, palladium, copper, nickel, tin, and lead in the form of fine grains. The conductive paste of claims 1, 2 or 3, wherein said conductive filler comprises copper with a surface oxygen concentration of 1.0 weight percent or less. The conductive paste of any one of claims 1 to 4, wherein said epoxy resin comprises at least one liquid epoxy resin selected from bisphenol A epoxy resins, bisphenol F epoxy resins, alicylic epoxy resins, amine epoxy resins and glycidyl esterificated epoxy resins. The conductive paste of any one of claims 1 to 5, wherein said liquid epoxy resin is a compound comprising 50 weight percent or less of one epoxy resin selected from bisphenol A and bisphenol F epoxy resins, and further comprising 50 weight percent or more of epoxy resin composed of glycidyl esterificated dimer acids with an epoxy equivalent of from 300 to 600 g/eq. The conductive paste of any one of claims 1 to 6, wherein said hardener is at least one hardener selected from amine hardeners, urea hardeners, acid anhydride hardeners, and aromatic amine hardeners. The conductive paste of any one of claims 1 to 7, wherein said hardener comprises a latent hardener. The conductive paste of any one of claims 2 to 8, wherein said dispersant comprises at least one dispersant selected from nonionic dispersants, anionic dispersants, and quarternary ammonium salt type cationic dispersants. The use of the conductive paste of any one of claims 1 to 9 for filling via holes in the insulating base material of printed circuit boards. A double sided printed circuit board comprising an insulating base material disposed with inner via holes which are filled with the hot-pressed, hardened conductive paste of any one of claims 1 to 9 wherein upper and lower electrode layers present on the surface of said insulating base material are electrically connected. A multilayer printed circuit board comprising a plurality of insulating base material layers and two or more electrode layers, wherein each insulating base material is disposed with inner via holes filled with the hot-pressed, hardened conductive paste of any one of claims 1 to 9 wherein each electrode layer is electrically connected. The printed circuit board as in claim 11 or 12, wherein said insulating base material comprises a composite material of an aramid fiber and a thermosetting resin. The printed circuit board as in claim 11 or 12, wherein said insulating base material comprises a composite material of an aramid nonwoven fabric and an epoxy resin. A method of manufacturing a double sided printed circuit board comprising the steps of: disposing a prepreg used for manufacturing a printed circuit board with via holes; filling the via holes with a conductive paste of any one of claims 1 to 9; placing said prepreg between copper foils on the upper and lower layer and providing heat and pressurization; and forming circuits by etching the copper foils. A method of manufacturing a multilayer printed circuit board comprising the steps of: disposing a prepreg used for manufacturing a printed circuit board with via holes; filling the via holes with a conductive paste of any one of claims 1 to 9; positioning said prepregs above and below a double sided printed circuit board; placing said prepregs between copper foils on the top layers, providing heat and pressurization, and etching the copper foils; or providing heat and pressurization and forming circuits. The method of claim 15 or 16, wherein said prepreg comprises a composite material of an aramid fiber and a thermosetting resin. The method of claim 15 or 16, wherein said prepreg comprises a sheet composed of an aramid nonwoven fabric impregnated with a thermosetting epoxy resin.

Inventors: AKIYAMA HIDEO, IWAISAKO KOUICHI, KAWAKITA KOUJI, NAKATANI SEIICHI, OGAWA TATSUO, SUEHIRO MASATOSHI, AKIYAMA, HIDEO,, IWAISAKO, KOUICHI,, KAWAKITA, KOUJI,, NAKATANI, SEIICHI,, OGAWA, TATSUO,, SUEHIRO, MASATOSHI,
Applicants: DAI ICHI KOGYO SEIYAKU CO LTD, DOWA MINING CO, MATSUSHITA ELECTRIC IND CO LTD, DAI-ICHI KOGYO SEIYAKU CO., LTD., DOWA MINING CO., LTD., MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1994
Main classification: H05K3
IPC codes: H05K1, H01B1, H05K3